A spectrometer is an optical device that separates light to be measured into spectral components by means of a dispersive element, such as a prism or a diffraction grating. With such a device, by detecting the spectral components of the light spectrally separated by the dispersive element, a wavelength distribution of the light, an intensity of a specific wavelength component, etc., can be made known. Such a device is thus used in various applications (see, for example, Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open No. H09-89669